Finned Seals for Turbomachinery

ABSTRACT

A seal assembly ( 50 ) controls leakage of working fluid through an annular gap (G) between a static component ( 16 ) and a rotary component ( 28 ) in a turbomachine. The fixed and moving components ( 16, 28 ) each have stepped diameters including a plurality of circumferentially and axially extending lands ( 56, 58 ) that confront each other across the annular gap (G). They are complementarily formed such that the annular gap is maintained over the axial extent of the seal assembly. Both components ( 16, 18 ) are provided with rows of fins ( 60, 62 ) which extend circumferentially of the lands and project radially therefrom towards each other. The rows of fins  60  of the static component  16  are preferably unequally spaced apart with respect to the rows of fins  62  of the rotating component  16 , so producing a vernier seal arrangement.

This application is a Continuation of, and claims priority under 35U.S.C. § 120 to, U.S. application Ser. No. 10/994,391, filed 23 Nov.2004, and claims priority therethrough under 35 U.S.C. § 119 to GreatBritain application number 0327300.0, filed 25 Nov. 2003, by theinventors hereof, the entireties of which are incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of Endeavour

The present invention relates to turbomachinery, and in particular toimprovements in finned seals, such as can be used to control flow offluids through clearances between stationary and rotating components.

2. Brief Description of the Related Art

In turbomachines, such as steam turbines, there is a need to controlleakage of the working fluid through annular gaps (clearances) betweenrotating and stationary components. One known means of controllingleakage of working fluid between rotating and stationary components isthe finned seal. In one form, this comprises an axial series ofcircumferentially extending ribs or fins which project from both thestationary and rotating components towards each other across the annulargap.

SUMMARY

One of numerous aspects of the present invention includes providing animproved finned seal that can minimise leakage through annularclearances between static and rotating components in turbomachinerywhile accommodating relative axial movement between such components.

Accordingly, another aspect of the present invention includes providinga seal assembly for controlling leakage of fluid through an annular gapbetween a rotary component and a static component in a turbomachine, inwhich the rotary and static components each have stepped diameterscomprising a plurality of circumferentially and axially extending landswhich confront each other across the annular gap and are complementarilyformed such that the annular gap is maintained over the axial extent ofthe seal assembly, both components being provided with rows of finswhich extend circumferentially of the lands and project radiallytherefrom towards each other, rows of fins on confronting lands beingopposed to each other across the gap, the radial dimensions of theopposed fins being sufficient substantially to span the gap when addedtogether.

The annular gap is exemplarily maintained substantially constant inradial dimension over the axial extent of the seal assembly. However,axially successive lands on both components may decrease in diameterstepwise over a first axial extent of the seal assembly and increase indiameter stepwise over a second axial extent of the seal assembly.Alternatively, axially successive lands on both components may increasein diameter stepwise over a first axial extent of the seal assembly anddecrease in diameter stepwise over a second axial extent of the sealassembly.

In an exemplary embodiment, each of one or more rows of fins on therotary component and/or the static component comprises a pair of axiallyadjacent fins of substantially equal radial extent. Alternatively, eachof one or more rows of fins on the rotary component and/or the staticcomponent may comprise axially adjacent multiple fins.

Further aspects of the invention will become apparent from a study ofthe following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic side elevation in broken-away axial section ofpart of a steam turbine, including a known type of finned seal;

FIG. 1B is a view within the dashed rectangle B of FIG. 1A,diagrammatically illustrating a vernier seal;

FIG. 2 is a view like FIG. 1B but of a vernier seal in accordance withthe invention; and

FIGS. 3 and 4 are views like FIG. 2, but showing alternative embodimentsof the invention;

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1A illustrates part of a steam turbine, comprising two annular rowsof moving blades 10, 12 and an annular row of fixed blades 14 betweenthe two rows of moving blades. At their radially inner ends, the fixedblades 14 are joined to an inner shroud ring 16. For convenience ofmanufacture and assembly, the shroud ring 16, termed the fixed shroud,may be formed as a number of circumferentially extending sectors of anannulus, or as a pair of half-rings. The radially outer surface 18 ofthe shroud 16 helps define the radially inner boundary of the turbinepassage 19.

At their radially inner ends, the moving blades 10, 12 are provided withroot portions 20, 22 by which they are attached to the rims ofrespective rotor discs 24, 26. As shown, the blade root portions 20, 22are of the re-entrant slot type, the slots having a sectional profilesomewhat like a fir-tree. Alternatively, other forms of attachments,such as pinned fingers or dovetails, could be used to secure the movingblades to the rotor discs. The rotor discs 24, 26 extend radially from acylindrical shaft 28.

During operation of the turbine, some of the steam from the turbineannulus 19 tends to leak around the radially inner end of the fixedshroud 16 (as indicated by the arrows) instead of flowing through thepassages between successive blades 14 in the static blade row. Tomaintain turbine efficiency, it is necessary to control this flow ofsteam and for this purpose a known type of labyrinth seal assembly 30 isprovided. This comprises a radially inner cylindrical surface 32 of thefixed shroud 16 that confronts an outer cylindrical surface 34 of theshaft 28 across a gap G. Extending radially inwards from the surface 32towards the shaft is an axial series of circumferentially extending finsor ribs 36; similarly, extending radially outwards from the surface 34towards the fixed shroud 16 is an axial series of circumferentiallyextending fins or ribs 38. Fins 36 and 38 are axially offset from eachother, so that they are interdigitated, thereby presenting steam with aserpentine path of increased flow resistance to reduce leakage.

It will be seen from FIG. 1A that the fins 36 and 38 do not extend allthe way across the gap G between the confronting surfaces 32, 34. Thisprevents the free ends of the fins rubbing against the confrontingsurfaces 32, 34.

Another type of finned seal 40 suitable for use in the turbine of FIG.1A is illustrated diagrammatically in FIG. 1B. It again comprises anaxial series of circumferentially extending fins or ribs 42, 44 providedon each confronting surface 32, 34 of the static and rotating components16 and 28, respectively. Fins 42, 44 extend circumferentially of thefixed shroud 16 and the shaft 28 and project radially towards eachother. There are the same number of fins 42, 44 on each of theconfronting surfaces 32, 34, each fin 42 being opposed to a fin 44across the gap G. The radial dimensions of the opposed fins do not haveto be identical, but when added together must be sufficient to span thegap G effectively, though of course during normal operation there shouldbe a small radial clearance between the free ends of the opposed fins.

It will be seen that some of the opposed fins, e.g., 42A, 44A, areoffset from each other across the gap G, while others, e.g., 42B, 44B,are in registration with each other. This is because the ribs 42 on thefixed shroud ring 16 are axially spaced apart from each other by aslightly different amount compared to ribs 44 on the shaft 28. This ischaracteristic of so-called vernier-type seals, which are designed suchthat under a defined range of axial positions of the rotating and fixedcomponents relative to each other, there is always at least one sealingrib or fin on one component in registration (or nearly so) with acorresponding rib or fin on the other component, so maintainingrestriction of fluid flow through the gap G.

The skilled person will realise that axial movement of a steam turbinerotor relative to the turbine's fixed structure will be due, e.g., todifferences in linear thermal expansion between the turbine casing andthe rotor, or movement of the rotor in its bearings due to thrust forcestransmitted from the turbine blades. The possible range of such axialmovement will be known from tests and/or calculation, and therefore thevernier seal will be designed to cope with this specific range ofmovement.

A disadvantage of the vernier seal of FIG. 1B is that if the gap G isreduced due, e.g., to differential thermal growth in the radialdirection between the shaft 28 and the fixed shroud 16, some of theopposing ribs 42 and 44 which happen to be in registration with eachother at the time will rub on each other and wear away. This will openup the existing small radial clearances between the free ends ofopposing ribs and thereby tend to increase the amount of leakage flowthrough the seal assembly. This is particularly so because unlike thelabyrinth seal of FIG. 1A, the flow of leakage fluid does not have toturn corners in order to get through the clearances, but can flowthrough the vernier seal of FIG. 1B in a straight line.

Turning now to FIG. 2, there is shown a seal assembly constructed inaccordance with the invention. A vernier seal assembly 50 again controlsleakage of fluid through the gap G between the fixed and movingcomponents, but unlike FIGS. 1A and 1B, the fin-bearing surfaces 52, 54of the fixed shroud 16 and the shaft 28 are not of constant diameter butare radially stepped. In this example, the stepped diameters form sevencircumferentially and axially extending lands 56 and 58 on the fixedshroud and the shaft, respectively, though more or fewer steps could beprovided. The lands confront each other across the annular gap G and thediameters of confronting lands are complementarily dimensioned withrespect to each other such that, though stepped, the gap G issubstantially constant over the axial extent of the seal assembly. Thelands on both the rotor 28 and the fixed shroud 16 are provided withrows of fins 60, 62 which extend circumferentially of the lands andproject radially therefrom towards each other such that rows of fins onconfronting lands are opposed to each other across the gap G. As in FIG.1B, the radial dimensions of the opposed fins are sufficient tosubstantially span the gap when added together.

It will be realised that the steps in diameter of the lands 56 and 58removes the ability of the leakage fluid to flow in a straight linethrough the seal, even when some of the fins have been shortened due torubbing against each other. Hence, the flow resistance of the seal isincreased relative to a “straight through” version of the seal withoutsteps.

As will be seen from FIG. 2, axially successive lands on both the rotorand the static shroud ring decrease in diameter stepwise over a firstaxial extent ‘A’ of the seal assembly and increase in diameter stepwiseover a second axial extent ‘B’ of the seal assembly.

From FIG. 2, it is evident that each row of fins 60, 62 in the sealassembly 50, on both the rotor and the fixed shroud ring, is in fact anaxially adjacent double fin 60A, 60B and 62A, 62B, having the sameradial extents. These double fins comprise the radially projecting freeends of circumferentially extending elongate components which have asubstantially U-shaped cross-section. Other arrangements of fins arepossible, such as rows comprising single or multiple fins. Furthermore,fins may be constructed as separate components, or be integral with theshroud ring or rotor. Conveniently, the elongate components are stripsembedded in grooves 64, 66 formed in the surfaces of the confrontinglands 54 and 56, the cross-sectional shape of the grooves beingcomplementary to the U-shaped cross-section of the strips. The stripsmay be made of any suitable material and are secured in the grooves bycaulking 67 or other suitable means.

The benefit of multiple rows of double fins as shown in FIG. 2, is anincrease in longevity of the seal and increased effectiveness over arange of axial movement of the rotor. Furthermore, the simple method ofmaking the fins and fixing them into the confronting surfaces of themoving and fixed components means that all the fins can be easilyrefurbished or replaced during an overhaul of the turbine.

As noted above, one or more of the rows could comprise single fins, thisbeing achieved by the simple expedient of having one limb of theU-shaped strips shorter than the other and level with the surface of theland in which it is embedded.

FIG. 3 shows an alternative arrangement for a seal assembly 70, in whichaxially successive lands 72, 74 on both the moving and static componentsincrease in diameter stepwise over a first axial extent ‘A1’ of the sealassembly and decrease in diameter stepwise over a second axial extent‘B1’ of the seal assembly.

It will be realised by the skilled person that the steps in thediameters of adjacent lands need not be equal increments or decrementsof diameter, though it will probably still be desirable to maintain aconstant radial dimension of the gap G over the axial extent of the sealassembly.

The vernier effect in the vernier seal assembly described above inrelation to FIGS. 2 and 3 may be obtained in a variety of ways. Thenormal way is that the rows of fins on both components are equallyspaced apart with respect to fins on the same component, but the spacingon one component differs slightly from the spacing on the other.Alternatively, the rows of fins on either or both components may beunequally spaced apart from each other to obtain an exaggerated verniereffect if such is deemed desirable.

Although a vernier seal arrangement is illustrated in FIGS. 2 and 3 ofthe accompanying drawings, it is envisaged that provided only a smallrange of axial movement of the rotating component is required to beaccommodated by the seal, the invention could also operatesatisfactorily without use of the vernier effect in spacing apart ofadjacent rows of fins. That is, as shown in FIG. 4, a seal 80 couldutilise spacing between the rows of fins 82, 84 which is identical overthe axial extents of the seal assembly and is the same on both thestatic and rotating components.

Although the focus of the above description has been on use of theinvention in connection with an axial flow steam turbine, the skilledperson will appreciate that the invention could be applicable to othertypes of turbomachinery, whether or not steam-driven, including radialflow turbomachines and including radial or axial flow compressors.

List of reference numbers.

-   -   10, 12—moving turbine blades    -   14—fixed turbine blades    -   16—inner fixed shroud ring    -   18—radially outer surface of shroud 16    -   19—turbine passage    -   20,22—root portions of rotor blades 10, 12    -   24, 26—rotor discs    -   28—shaft    -   30—labyrinth seal    -   32—inner cylindrical surface of fixed shroud 16    -   34—outer cylindrical surface of shaft 28    -   36, 38—fins on surfaces 32, 34    -   40—vernier seal    -   42, 44—fins    -   42A, 44A—opposed fins offset from each other    -   42B, 44B—opposed fins in registration with each other    -   50—vernier seal (invention)    -   52, 54—confronting surfaces of fixed shroud 16 and shaft 28    -   56, 58—lands    -   60, 62—fins on lands 56, 58    -   64, 66—grooves    -   67—caulking    -   70, 80—seal assembly    -   82, 84—fins in seal assembly 80    -   ‘A’, ‘B’—first and second axial extents of the seal assembly in        FIG. 2    -   ‘A1’, ‘B1’—first and second axial extents of the seal assembly        in FIG. 3    -   G—gap

While the invention has been described in detail with reference toexemplary embodiments thereof, it will be apparent to one skilled in theart that various changes can be made, and equivalents employed, withoutdeparting from the scope of the invention. The foregoing description ofthe preferred embodiments of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto, and theirequivalents. The entirety of each of the aforementioned documents isincorporated by reference herein.

1. A seal assembly for controlling leakage of fluid through an annulargap between a rotary component and a static component in a turbomachine,comprising: a rotary component and a static component each havingstepped diameters comprising a plurality of circumferentially andaxially extending lands which confront each other across an annular gaptherebetween and are complementarily formed such that the annular gap ismaintained over an axial extent of the seal assembly, both componentsincluding a plurality of fins arranged on the lands in axial successionto each other, the fins extending circumferentially of the lands andprojecting radially from said lands towards each other, fins onconfronting lands being opposed to each other across the gap, the radialdimensions of the opposed fins being sufficient substantially to spanthe gap when added together; and wherein at least one fin on one of thecomponents is in registration with a corresponding opposed fin on theother component.
 2. A seal assembly according to claim 1, wherein theannular gap is substantially constant in radial dimension over the axialextent of the seal assembly.
 3. A seal assembly according to claim 1,wherein at least one of the fins on the rotary component, on the staticcomponent, or both, comprises a double fin.
 4. A seal assembly accordingto claim 3, further comprising: circumferentially extending elongatecomponents having a substantially U-shaped cross-section; and whereinthe double fin comprises radially projecting free ends of thecircumferentially extending elongate components.
 5. A seal assemblyaccording to claim 4, wherein the static component comprises a staticshroud ring; further comprising grooves formed in the confronting landson the rotor and on the static shroud ring; and wherein thecircumferentially extending elongate components are secured in thegrooves in the confronting lands on the rotor and on the static shroudring, the cross-sectional shape of the grooves being complementary tothe U-shaped cross-section of the elongate components.
 6. A sealassembly according to claim 1, wherein at least one of the fins on therotary component, on the static component, or both, comprises a multiplefin.
 7. A seal assembly according to claim 6, further comprising:circumferentially extending elongate components having a substantiallyU-shaped cross-section; and wherein the multiple fin comprises radiallyprojecting free ends of the circumferentially extending elongatecomponents.
 8. A seal assembly according to claim 7, wherein the staticcomponent comprises a static shroud ring; further comprising groovesformed in the confronting lands on the rotor and on the static shroudring; and wherein the circumferentially extending elongate componentsare secured in the grooves in the confronting lands on the rotor and onthe static shroud ring, the cross-sectional shape of the grooves beingcomplementary to the U-shaped cross-section of the elongate components.9. A seal assembly according to claim 1, wherein the spacing betweenaxially successive fins on the rotary component differs from the spacingbetween axially successive fins on the static component to produce avernier seal arrangement.
 10. A seal assembly according to claim 1,wherein the spacing between axially successive fins on either or bothcomponents is unequal.
 11. A seal assembly according to claim 1, whereinboth components have axially successive lands comprising one of astepwise decrease in diameter over a first axial extent of the sealassembly and a stepwise increase in diameter over a second axial extentof the seal assembly, and a stepwise increase in diameter over a firstaxial extent of the seal assembly and a stepwise decrease in diameterover a second axial extent of the seal assembly.
 12. A seal assemblyaccording to claim 1, wherein a plurality of fins on one of thecomponents is in registration with a corresponding plurality of opposingfins on the other component.
 13. A seal assembly according to claim 1,wherein a plurality of fins on one of the components is offset from acorresponding plurality of opposing fins on the other component.
 14. Aseal assembly according to claim 1, wherein all the fins on one of thecomponents are in registration with corresponding opposing fins on theother component.